High density magnetic recording is highly sought after in advanced information societies these days. In place of a conventional longitudinal magnetic recording system, a perpendicular magnetic recording system is in practical use as a technology to achieve high density.
In the perpendicular magnetic recording system, a magnetic layer of a perpendicular magnetic recording medium is formed such that an axis of easy magnetization is oriented perpendicular to a medium plane. The system is suitable for high density recording due to a small demagnetizing field in a bit, even at an increased recording density, and a limited reduction in read/write properties. For the perpendicular magnetic recording system, a recording medium is developed having a magnetic recording layer and a soft magnetic under layer with increased recording sensitivity.
The soft magnetic under layer, which returns a recording magnetic field from a magnetic head, must have a high saturation magnetic flux density to efficiently draw in a recording magnetic field and must have a high magnetic permeability to enhance writability to a magnetic recording medium (refer to Patent Literatures 1 and 2, for example). High surface roughness of the soft magnetic under layer leads to high surface roughness of the perpendicular magnetic recording medium, thus increasing flying height of the magnetic head and deteriorating reproducing sensitivity. Thus, the soft magnetic under layer must have an amorphous structure having excellent surface smoothness (refer to Patent Literature 3, for example).
A soft magnetic under layer is suggested having a structure in which a plurality of soft magnetic layers are antiferromagnetically coupled through non-magnetic spacer layers inserted therebetween. The structure prevents a leakage magnetic flux generated from a magnetic wall of the soft magnetic layer from flowing into a reproducing head and fixates the magnetic wall existing in the soft magnetic layer so as not to be easily moved, thus reducing noise (refer to Patent Literature 4, for example). In such a soft magnetic under layer, a material having a high saturation magnetic flux density is suggested for use in the soft magnetic layer to enhance writability. Antiferromagnetic coupling within a range of 30% to 70% of a maximum value of an antiferromagnetic coupling force, which varies depending on the thickness of the spacer layer inserted between the soft magnetic layers, is suggested for improving high-speed response to high-frequency signals (refer to Patent Literature 5, for example).
A Co—Fe—Al alloy or the like is suggested as a material for the soft magnetic layer of the conventional perpendicular magnetic recording medium (refer to Patent Literatures 6 to 8, for example).